Download Chapter 3 Mendelian Genetics

Experimental Approach
0 Model of experimental design.
0 Selection of model organism
0 Pea plant easy to grow
0 Easily self-fertilizes
0 Easy to hybridize
0 Matures in single season.
0 Restricted examination to single or
few traits with distinct variations.
0 Terrific record-keeping of data.
0 Postulates generated provided
invaluable principles for
transmission genetics.
Mendel’s Traits
P, F1 and F2 Generations
0 By performing a MONOHYBRID cross, Mendel compared ONE pair of
contrasting traits.
0 1st: He selected a TALL pure bred plant and fertilized it with a SHORT pure
bred plant; he termed this the Parent (P) generation.
0 2nd: He observed offspring in the first Filial (F1) generation to be all TALL
plants.
0 3rd: He allowed the F1 generation to SELF-FERTILIZE and observed offspring
in the second Filial (F2) generation to ne ¾ TALL and ¼ SHORT
Important Information
0 Reciprocal crosses:
hybridization was performed
with pollen and ovum for all
crosses to prove that pollen
or ovum didn’t play a role in
expression of the trait.
0 Mendel performed an
enormous number of tests,
nearly 20,000 pieces of data
collected on the 7
observable characteristics.
0 Of this data, Mendel was
able to notice patterns in
the data.
0 He used the consistency of
the data to generate his first
3 postulates.
Mendel’s Postulates
#1: Unit Factors Occur in Pairs
MENDEL’S WORDS
o Genetic characters are
controlled by unit factors
that exist in pairs in
individual organisms.
MODERN TERMS
o Characteristics are
controlled by genes that
exist in pairs in individual
organisms.
Mendel’s Postulates
#2: Dominance/Recessiveness
MENDEL’S WORDS
o When two unlike unit factors
responsible for a single
character are present in a
single individual, one unit
factor is dominant to the
other, which is said to be
recessive.
MODERN TERMS
o When two unlike alleles
responsible for a single
character are present in a
single individual, one allele
is dominant to the other,
which is said to be recessive.
Mendel’s Postulates
#3: Segregation
MENDEL’S WORDS
o During the formation of
gametes, the paired unit
factors separate or segregate
randomly so that each gamete
receives one unit factor or
the other with equal
likelihood.
MODERN TERMS
o During the formation of
gametes, the paired alleles
separate or segregate
randomly so that each
gamete receives one allele or
the other with equal
likelihood.
Other Words to Know
0 Genotype: genetic make-up of an individual; designated
using capital or lowercase letters.
0 Phenotype: physical appearance of a trait; typically an
adjective.
0 Gene: units of inheritance/information for the order of amino
acids for specific protein construction.
0 Allele: alternative forms of a gene; options. For Mendel’s pea
plants, each gene had 2 alleles.
0 Homozygous: identical alleles; CC or cc
0 Heterozygous : not identical alleles; Cc
Testcross: One Character
0 Used to distinguish the GENOTYPE
of an organism with a DOMINANT
PHENOTYPE.
0 Crossing an UNKNOWN DOMINANT
(PP or Pp) organism with a KNOWN
HOMOZYGOUS RECESSIVE (pp).
The Predictability of
Monohybrid Crosses
Cross
Genotype
Phenotype
PP x PP
100% PP
100% purple
pp x pp
100% pp
100% white
PP x pp
100% Pp
100% purple
Pp x Pp
25% PP, 50% Pp, 25% pp
1:2:1
75% purple, 25% white
3:1
Mendel’s Postulates
#4: Independent Assortment
MENDEL’S WORDS
o During the formation of
gametes, segregating pairs of
unit factors assort
independently of each other.
MODERN TERMS
o During the formation of
gametes, segregating pairs of
genes/alleles assort
independently of each other.